include/idx.h

   1 /* A type for indices and sizes.
   2    Copyright (C) 2020-2021 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #ifndef _IDX_H
  20 #define _IDX_H
  21
  22 /* Get ptrdiff_t.  */
  23 #include <stddef.h>
  24
  25 /* Get PTRDIFF_MAX.  */
  26 #include <stdint.h>
  27
  28 /* The type 'idx_t' holds an (array) index or an (object) size.
  29    Its implementation promotes to a signed integer type,
  30    which can hold the values
  31      0..2^63-1 (on 64-bit platforms) or
  32      0..2^31-1 (on 32-bit platforms).
  33
  34    Why a signed integer type?
  35
  36      * Security: Signed types can be checked for overflow via
  37        '-fsanitize=undefined', but unsigned types cannot.
  38
  39      * Comparisons without surprises: ISO C99 § 6.3.1.8 specifies a few
  40        surprising results for comparisons, such as
  41
  42            (int) -3 < (unsigned long) 7  =>  false
  43            (int) -3 < (unsigned int) 7   =>  false
  44        and on 32-bit machines:
  45            (long) -3 < (unsigned int) 7  =>  false
  46
  47        This is surprising because the natural comparison order is by
  48        value in the realm of infinite-precision signed integers (ℤ).
  49
  50        The best way to get rid of such surprises is to use signed types
  51        for numerical integer values, and use unsigned types only for
  52        bit masks and enums.
  53
  54    Why not use 'size_t' directly?
  55
  56      * Because 'size_t' is an unsigned type, and a signed type is better.
  57        See above.
  58
  59    Why not use 'ptrdiff_t' directly?
  60
  61      * Maintainability: When reading and modifying code, it helps to know that
  62        a certain variable cannot have negative values.  For example, when you
  63        have a loop
  64
  65          int n = ...;
  66          for (int i = 0; i < n; i++) ...
  67
  68        or
  69
  70          ptrdiff_t n = ...;
  71          for (ptrdiff_t i = 0; i < n; i++) ...
  72
  73        you have to ask yourself "what if n < 0?".  Whereas in
  74
  75          idx_t n = ...;
  76          for (idx_t i = 0; i < n; i++) ...
  77
  78        you know that this case cannot happen.
  79
  80        Similarly, when a programmer writes
  81
  82          idx_t = ptr2 - ptr1;
  83
  84        there is an implied assertion that ptr1 and ptr2 point into the same
  85        object and that ptr1 <= ptr2.
  86
  87      * Being future-proof: In the future, range types (integers which are
  88        constrained to a certain range of values) may be added to C compilers
  89        or to the C standard.  Several programming languages (Ada, Haskell,
  90        Common Lisp, Pascal) already have range types.  Such range types may
  91        help producing good code and good warnings.  The type 'idx_t' could
  92        then be typedef'ed to a range type that is signed after promotion.  */
  93
  94 /* In the future, idx_t could be typedef'ed to a signed range type.
  95    The clang "extended integer types", supported in Clang 11 or newer
  96    <https://clang.llvm.org/docs/LanguageExtensions.html#extended-integer-types>,
  97    are a special case of range types.  However, these types don't support binary
  98    operators with plain integer types (e.g. expressions such as x > 1).
  99    Therefore, they don't behave like signed types (and not like unsigned types
 100    either).  So, we cannot use them here.  */
 101
 102 /* Use the signed type 'ptrdiff_t'.  */
 103 /* Note: ISO C does not mandate that 'size_t' and 'ptrdiff_t' have the same
 104    size, but it is so on all platforms we have seen since 1990.  */
 105 typedef ptrdiff_t idx_t;
 106
 107 /* IDX_MAX is the maximum value of an idx_t.  */
 108 #define IDX_MAX PTRDIFF_MAX
 109
 110 /* So far no need has been found for an IDX_WIDTH macro.
 111    Perhaps there should be another macro IDX_VALUE_BITS that does not
 112    count the sign bit and is therefore one less than PTRDIFF_WIDTH.  */
 113
 114 #endif /* _IDX_H */